Mutants that affect the de novo purine biosynthetic pathway, and the purine salvage enzymes have been isolated in multiply marked strains of CHO cells, e.g., Pro Pur4 AzgR alpha-amR. Such mutants fall into two classes on the basis of mutagen used. ICR-170 induced mutants resistant to Azguanine or Diaminopurine have no detectable HGPRT or APRT respectively. Mutants induced by EMS or spontaneously do have residual enzyme activity. The effect of mutation in the HGPRT locus on de novo purine biosynthetic pathway is being investigated. Depending on nutritional conditions, some (but not all) mutants show elevated purine biosynthesis -- 8-10 times parental levels. Other mutants do not show any elevation, and one mutant shows decreased levels of FGAR biosynthesis. Initial results would indicate that the substrate levels of various components of the de novo pathway have been altered as a result of a primary lesion in HGPRT. This will be investigated further by measuring the effects of glycine and glutamine on purine biosynthesis in the mutant and wild type. We shall also look for a correlation between glutamine starvation and PRPP levels. Similar studies will be performed with APRT mutants. Attempts will be made to introduce E. coli APRT (or HGPRT) onto phage to be used in studying gene transfer phenomenon.